1. Field of the Invention
The present invention relates to an antenna for communicating with a low earth orbit satellite, particularly relates to an antenna for communicating with a low earth orbit satellite used for an earth station in a satellite communication system in which plural low earth orbit (LEO) satellites revolve around the earth for automatically tracking each satellite.
2. Description of the Related Art
Recently, a scheme that high-speed data at approximately a few Mbps to a few tens Mbps is provided to users all over the world using a high-frequency signal in Ka band (20 to 30 GHz) via multiple LEO satellites is formed.
In such a satellite communication system using multiple LEO satellites, as each satellite goes off a visual field in relatively short time when viewed from a small-sized earth station, it is required to be tracked in a large range.
Heretofore, for an antenna for tracking a satellite, plural techniques are widely known as the antenna of an earth station for a geostationary satellite and a mobile satellite.
For example, for a method of tracking, a monopulse tracking method of continuously detecting whether an antenna tracks a satellite in the center of a beam or not and controlling so that an antenna bearing is always equal to the bearing of a satellite, a step tracking method of shifting an antenna at a fixed interval of time by degrees and adjusting it in a bearing in which a receiving level is maximum and a program tracking method of changing the bearing of an antenna based upon the estimated information of a satellite orbit are known.
For a method of supporting a mobile antenna, Az-EL mounting in which the azimuth angle and the elevation angle of the mobile antenna are shifted and XY mounting which the mobile antenna is shifted in a direction perpendicular to a satellite orbital direction are widely known. The Az-EL mounting is currently the most adopted method, one axis (the azimuth axis) is arranged perpendicularly to the ground and the other axis (the elevation axis) is arranged horizontally. In the XY mounting, the x-axis horizontal with the ground is perpendicular to the y-axis and the y-axis is turned together with the x-axis. The XY mounting is suitable for tracking a LEO satellite which moves near the zenith at high speed, however, as both axes are located in high positions from the ground, the XY mounting has a mechanical defect.
Next, referring to the drawings, the satellite tracking technique of an antenna of a conventional type concrete earth station will be described.
FIG. 13 shows the constitution of a conventional type antenna of an earth station for tracking a satellite. FIG. 13 shows an example of a large-sized antenna of an earth station for tracking a satellite and the main reflector is Cassegrainian antenna 13 m in diameter. The antenna tracks a satellite using a driving mechanism according to Az-EL mounting, and both the azimuth axis and the elevation axis are driven by a jackscrew driving mechanism. To simplify structure, the driving mechanism is allowed to continuously drive only within a range of .+-.10.degree. in the direction of an azimuth and a limited driving method that when an antenna is required to be directed at a larger angle in another direction, a set screw is loosened and the antenna is turned slowly is adopted. For the elevation axis, continuous driving between 0.degree. and 90.degree. is enabled. A primary feed is attached to the main reflector and is integrally driven with the main reflector.
FIG. 14 shows another conventional type antenna of an earth station for tracking a satellite and a small-sized antenna of an earth station for tacking a satellite in which miniaturization and lightening are realized though an aperture antenna is used as the above large-sized antenna is known.
FIG. 14 shows a parabolic antenna used for a ship earth station according to International Maritime Satellite Organization (INMARSAT) standard A, and a cross dipole and a reflector are located in the focus of a reflector with a paraboloid as a primary feed. In the antenna, the reflector and the feed are also integrated. To track a satellite, the above parabolic antenna is driven using four-axes mounting obtained by combining the above Az-EL mounting and XY mounting.
The above technique is described in "Guide to maritime satellite communication" written by Mr. Toshio Sato and published on Jul. 25, 1986 by Institute of Electronics and Communication Engineers of Japan.
As described above, technique for tracking a satellite used for the conventional type antenna for satellite communication can be effectively applied to a case in which a tracking range is relatively small as a geostationary satellite, however, the above conventional type antenna is not suitable for the above antenna for tracking and communicating with a LEO satellite for the following reasons:
That is, in the conventional type antenna for satellite communication, as the primary feed and the reflector are integrated and turn an antenna in tracking a satellite, the antenna to be turned is heavy, a driving system is also large-sized, high-speed tracking is difficult and the area of a radome for housing the antenna is also increased. In a satellite communication system using LEO satellites, considering that multiple small-sized earth stations are installed in each home and others, the size of the whole antenna is required to be as small-sized as possible and as light as possible, and miniaturization and lightening are a large problem.
Further, as the primary feed and the reflector are integrated and turn an antenna, a feeding system is required to be provided so that a radio frequency (RF) sending/receiving part such as a low noise amplifier and a high-frequency power amplifier is also mounted near the primary feed so as to stably feed to the primary feed also during turning, however, in this case, the weight of the antenna is also increased by the weight of the RF sending/receiving part.
In this case, it is also conceivable that the RF sending/receiving part is separated from the reflector and fixed, however, to maintain stable connection independent of the displacement by turning of the feeding part, an electric supply line is required to be flexible, a rotary joint and others are required to be used and there is a problem that an antenna for satellite communication is complicated and high-priced.
When a satellite being tracked in a certain orbit disappears from the north to the south because LEO satellites revolve in plural orbits, another satellite revolving in the same orbit is required to be tracked next. In this case, information communicated using the former satellite is required to be communicated using the latter satellite and hand over for instantaneously switching to the latter satellite is required.
However, the above conventional type technique has a problem that it is difficult to provide hand over for switching to another satellite in the same orbit.